1. Field of the Invention
This invention relates generally to a method for producing conductive or insulating feedthroughs from a front surface of a substrate to a back surface of the substrate, and to the substrate with feedthroughs produced from that method. Such feedthroughs make it possible to interconnect devices on opposite sides of the substrate or to interconnect devices on two different substrates without having to pass a wire around an outer edge of the substrate.
2. Description of Related Art
Components on opposite sides of a single substrate or on two separate substrates in a multilayer integrated circuit must often be interconnected. This can be accomplished by passing a wire from one of the components, around the substrate to the other component. However, since electric signals travel from one point to another at finite speeds, the lengths of interconnections between such components has become an important parameter in determining the final rate of operation of multilayer integrated circuits. It is therefore desirable to provide short interconnections between such devices.
One way to provide shorter interconnections is to fabricate feedthroughs directly in substrates. FIGS. 1a and 1b are examples of two situations in which separate components must be interconnected and the components are on opposite sides of a single substrate and on different substrates, respectively. In particular, Figure la shows a substrate 1 with a component 2 on a first surface 3 and a component 4 on a second surface 5 of substrate 1, whereas FIG. 1b shows substrate 1 with component 2 on first surface 3 but with component 4 on a first surface 9 of another substrate 8. In both cases, if substrate 1 has no feedthrough, a wire 6 must be led from component 4 around substrate 1 to component 2 in order to interconnect components 2 and 4. If, on the other hand, substrate 1 does have feedthrough 7, a significantly shorter wire 6' can interconnect devices 2 and 4.
Surfaces 3, 5 and 9 can be conductive or non-conductive and feedthrough 7 can be an insulated feedthrough or a conductive feedthrough. If feedthrough 7 is an insulated feedthrough, then wire 6' is a single piece of wire which is passed (fed) through feedthrough 7. If, however, feedthrough 7 is a conductive feedthrough, then wire 6' is two pieces, one piece being connected to each end of feedthrough 7. Also, instead of using wire 6, conductive paths can be photolithographically fabricated directly onto surfaces 3, 5 or 9.
Another important feature for multiple layer circuitry is the diameter of feedthroughs. Larger diameter feedthroughs waste surface area of a substrate, thereby reducing the number of components which can be supported by the substrate. That is, if feedthroughs are produced by drilling holes into a substrate as described in "Machining of Advanced Ceramics," Laurel M. Sheppard, pp. 40-48, Advanced Materials and Processes Incorporating Metal Progress, Dec., 1987, the diameters of resulting drilled holes can be on the order of millimeters. This creates a mismatch between the size of integrated circuit components (typically in micrometers) and the size of the feedthroughs made by drilling holes. Similarly, a mismatch can occur between photolithographically created conductive paths and feedthroughs made by drilling holes. Therefore, it is also desirable to fabricate conductive feedthroughs in a substrate on the order of a few micrometers.
Another important feature for multiple layer circuitry is that it sometimes necessary to interconnect multi-conductor transmission lines (e.g., two conductor microstrip transmission lines) which are on two sides of a single substrate or on separate substrates. Therefore, it is also desirable to fabricate small diameter multi-conductor feedthroughs in a substrate.